1. Field of the Invention
The present invention relates to a zoom lens system and an image pickup apparatus using the zoom lens system.
2. Description of the Related Art
A digital camera is configured to photograph a subject by use of an electronic image pickup element such as a CCD image sensor or a CMOS image sensor instead of a silver halide film. In recent years, people have come to like a small and thin type of such digital camera. The thickness of the camera is determined mainly by the size of an optical system. Therefore, to achieve thinning of the camera, a constitution of the optical system is important. In recent years, a so-called collapsible lens barrel has become popular in which the optical system is projected from a camera body during photographing and in which the optical system is stored in the camera body when being carried. Therefore, in a zoom lens system, a constitution of each lens unit in consideration of the size when collapsed is important.
On the other hand, a zoom ratio of the compact digital camera is generally about 3×, but a camera having a higher zoom ratio has been demanded.
As a related technology suitable for such a zoom lens system having the high zoom ratio, a zoom lens system is known which has, in order from an object side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power and a fourth lens unit having a positive refractive power.
The zoom lens system having the above refractive power layout is disclosed in Japanese Patent Application Laid-Open Nos. 4-171,411 and 2005-62,228.
In Japanese Patent Application Laid-Open Nos. 2003-98,433 and 2004-199,000, a zoom lens system having the above refractive power layout is disclosed in which the first lens unit is constituted of one positive lens and second lens unit is constituted of two lenses including a negative lens and a positive lens in order to reduce cost and decrease a diameter.
In Japanese Patent Application Laid-Open No. 11-316,340, a zoom lens system having the above refractive power layout is disclosed in which the third lens unit is constituted of three lenses including a positive lens, a negative lens and a negative lens in order from an object side, and a principal point of the third lens unit is disposed closer to the object side. In this zoom lens system, a principal point of the third lens unit can be disposed closer to the second lens unit in a telephoto end. Therefore, a magnification change function of the third lens unit can be improved, and a zoom ratio of about 5× is realized.